1. Field of the Invention
This invention relates to a color picture tube having a plurality of electron guns and a magnetic focusing system.
2. Description of Prior Art
Japanese Utility Model Publication No. 26274/75 discloses a picture tube with a single electron gun having means provided on the outer or inner surface of the tube neck for generating a magnetic field for beam focusing, and a pair of members of magnetic material high in magnetic permeability each having one through hole for passing one electron beam inside the tube where the beam-focusing magnetic field is present, the members of magnetic material being spaced to each other in the direction of beam travel, i.e., along the tube axis so that the focusing magnetic field is strengthened on the one hand and a focusing magnetic lens with orderly magnetic field distribution is formed on the other hand between the pair of the members of magnetic material high in magnetic permeability.
This focusing system is such that the pair of members of magnetic material absorb and discharge the lines of magnetic force for concentration thereof, thus stengthening the magnetic field between the members of magnetic material, and therefore a magnetic field-generating device is available which is small in shape and weight, i.e., small in magnetomotive force on the one hand and by improving the dimensional accuracy of the members of magnetic material, the accuracy of the magnetic lens, is improved thus making it possible to use a magnetic field-generating device having a distribution of magnetic field low in accuracy on the other hand, leading to a superior picture tube of magnetic focusing type.
In application thereof to a color picture with a plurality of electron guns, however, such a focusing system has the disadvantages as mentioned below.
Assume, for instance, that such a focusing system is used with a color picture tube of 20-inch, 110.degree.-deflection type with neck diameter of 29 mm and having three in-line electron guns horizontally arranged and spaced 6.6 mm from each other. The focal length of the focusing magnetic lens is substantially equal to the distance from the crossover point near the first grid and the center of the magnetic lens, which distance is much shorter than the distance from the center of the magnetic lens to the phosphor screen 31. As a result, the three electron beams that have passed the magnetic lens cross each other at a point far from the phosphor screen 31 and are spaced so widely from each other on the phosphor screen 31 that the center beam is distant about 40 mm from the side beams on the phosphor screen 31. Further, the physical positions of the three electron guns are so distant from each other that when the travel route of the electron beam of one of the electron guns is determined along the lens axis, the other two electron beams pass along the margin of the magnetic lens, with the result that the astigmatism and coma for the particular two electron beams are increased, thus leading to the deformation of beam spots.